[wilecoyote]

I have a 2003 ZX3 and my brother liked it so much he bought one also. He now has a misfire at idle and when he turns on the a/c the problem gets real bad. To the point, he has to pump the accelerator to get the car to go from a dead stop. Anybody have any ideas on what it might be?

 

[BMARTIN03ZX3]

Is the CEL on? Also is your brothers the same year if it's a 2002.5 and older there is a recall on the fuel pump for 10 years or 100,000 miles, but I would check the plugs and wires also check behind the intake manifold and there's a pcv line the goes on the back of the intake and the vaccum lines get oil soaked after awhile and the hose cracks.

 

[viney266]

Pull the plugs and change them if they look iffy. If one looks more fouled, there is your issue, follow the problem from there.

 

[wilecoyote]

My brother's focus is a ZX3 2003 2.3 also. He has changed the plugs and wires. I don't understand how the a/c can affect misfire so drastically. Wouldn't the engine light come on?

 

[RhodySVT]

Turning on the a/c increases the load on the engine and usually it will compensate by raising the RPM a couple 100 to run smoother. With the misfire it probably can't compensate and runs even worse. Since he changed plugs/wires, I'd look at the coil and for a vac leak next.

 

[penguin]

> I don't understand how the a/c can affect misfire so drastically

It really isn't affecting the misfire at all, but rather the engine shaking and you perception of the misfire. With little load, the engine crankshaft doesn't slow-down that much when a single cylinder doesn't fire properly. But if you add load to the engine/crankshaft, there will be more of a slow-down when a cylinder misfires, and more of a subsequent pick-up when the next cylinder in the firing order fires.

So as you increase load on the engine, the misfire will become more and more noticeable.

> Wouldn't the engine light come on?

It should. But to avoid false alarms, some OBD-II systems doe not give a light upon the first notice of misfire, but require a sustained misfire over a period of time before lighting the CEL/MIL.

Most OBD-II systems monitor for misfires using the crankshaft position sensor to detect changes in crankshaft speed and associate the time of the change to a particular cylinder. But since other conditions, such as driving on a rough road, can also make sudden/momentary changes in the crankshaft speed, most require the data to fit a particular pattern over some period of time before a MIL light indication is given.

A persistent misfire should result in a CEL/MIL, but (a) not always immediately, and (b) only if the misfire continues for a period of time.

 

[wilecoyote]

Thanks everybody for your help.